Theoretically it can be any level. However, the more slots you have open the more chances you have of recruiting higher levels. Mine is 16, so I have all 6 slots open. However, the epic historic are extremely rare, and the developers would rather you pay for the packs to get high enough level to complete the full multi-day events. Since you usually need to be able to train thousands of T13 troops to do so, your keep and training sites need to be level 32 ($$$$$$$). Patience is required otherwise.

What is the best troop

They give boost percentages to different troop types. For example, Bird of Hurricane boosts mounted troops. Boosts apply to attack, defense and HP